Imaging devices, such as printers, facsimile machines, etc., often employ a print head for printing on a printable medium, such as paper. Ink is usually supplied to the print head from an ink reservoir via a flow passage. In one application, the ink reservoir and print head form a single unit, e.g., a print cartridge, and ink flows from the ink reservoir to the print head via the flow passage during printing. In another example, the ink reservoir and print head are separate, and during printing, ink flows from the ink reservoir to the print head via a flexible duct interconnecting the ink reservoir and the print head. Many print heads, such as used in ink-jet devices, include resistors that vaporize the ink supplied to the print head. This causes the ink to be ejected through orifices of the print head so as to print dots of ink on the printable medium.
To prevent ink leakage from the reservoir, it is common to exert a force on the ink to retain the ink within the ink reservoir. For example, many ink reservoirs contain a porous medium, such as foam (or an ink sponge), that is capable of absorbing and retaining ink. The capillarity of the ink sponge exerts a force (capillary force) that draws the ink into the ink sponge, preventing the ink from leaking out of the ink sponge and thus the reservoir. It is also common to use bladders within ink reservoirs for exerting retaining forces on inks.
Many ink reservoirs are vented to atmospheric pressure to prevent excessive vacuum pressures within the reservoir that can reduce or prevent ink flow to the print head. In addition, venting relieves pressure buildups that can occur when an ink reservoir is exposed to extreme environmental conditions, e.g., that can be encountered during shipping, such as high temperatures in motor vehicles or low pressures in airplanes at high altitudes.
Some ink reservoirs use porous membranes for venting. These membranes allow air to flow through the vent while acting to prevent the ink from leaking through the vent. However, the pores of these membranes can become blocked when wetted by ink. Moreover, many recently developed types of ink have reduced surface tensions (and thus better wetting capabilities) for enabling increased ink delivery rates for faster printing. To prevent these inks from wetting the membranes, ink reservoirs are usually under filled, e.g., so that only about 50 to 70 percent of the ink sponge is wetted by ink. Therefore, if the ink expands or the reservoir is over turned, the remaining 30 to 50 percent of the ink sponge will absorb the ink to prevent the ink from contacting the porous membrane. To further prevent these inks from wetting the membranes, the capillarity of the ink sponge is frequently increased for exerting a greater capillary force on the ink to better retain the ink within the ink sponge. One problem with this is that less ink can be extracted from high-capillarity ink sponges than lower-capillarity ink sponges for printing. This means that when ink can no longer be extracted from an ink sponge for printing, there is more undeliverable ink retained (stranded ink) within high-capillarity ink sponges than lower-capillarity ink sponges.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternatives for venting ink reservoirs.